The etiological factors and exposures that underlie colorectal cancer development are complex. Nevertheless, in the past decade, clearcut insights into the role of inherited and somatic mutations in colorectal cancer have been obtained. The stepwise accumulation of mutations in oncogenes, such as K-ras, and tumor suppressor genes, such as the APC and p53 genes, is believed critical in initiating adenomatous polyp formation and subsequent progression to fully invasive and metastatic cancer. Despite the considerable progress, additional mutations that underlie colorectal cancer development await discovery. Moreover, much work remains if we hope to understand the means by which specific mutations contribute to the cancer cell phenotype. Recent studies indicate that germline inactivation of one allele of an intestinal-specific homeobox gene, known as Cdx2, strongly predisposes mice to the development of adenomatous polyps and cancers in the small intestine and colon. We are particularly interested in determining the biological relevance of this finding with respect to human colorectal cancer. Our preliminary studies indicate that reduced or absent CDX2 expression is seen in a significant fraction of colorectal cancer; CDX2 mutations are present in some cases; and CDX2 alterations appear to arise independently of other gene defects. We have identified several candidate Cdx2-binding proteins that may allow us to define the growth regulatory pathway(s) in which CDX2 functions. The studies proposed in this application will further explore the nature and significance of CDX2 pathway abnormalities in human colorectal cancer. Four specific aims are proposed: I) To carry out comprehensive CDX2 mutational analyses and expression studies in colorectal cancer cell lines, xenografts, and primary tumors; II) To define mechanisms underlying CDX2 gene expression defects in cancer; III) To determine if CDX2 functions as a suppressor of in vitro growth and/or tumorigenesis, and if so, to define CDX2- regulated target genes; and IV) To identify Cdx-2 interacting proteins and address their role in cancer development. In addition to furthering our understanding of colorectal cancer pathogenesis, the studies in our application may offer novel insights for improving the diagnosis and treatment of patients with colorectal cancer.